The present invention generally relates to high-speed data communications on a transmission line. More specifically, the invention relates to an improved line driver with active line termination, which solves problems associated with power efficiency, recovering receive signals from a duplex signal transmission, and manufacturing variances of circuit components.
With the advancement of technology, and the need for instantaneous information, the ability to transfer digital information from one location to another, such as from a central office (CO) to a customer premise (CP) has become more and more important.
In a digital subscriber line (DSL) communication system, and more particularly an xDSL system where xe2x80x9cxxe2x80x9d indicates a plurality of various standards used in the data transfer, data is transmitted from a CO to a CP via a transmission line, such as a two-wire twisted pair, and is transmitted from the CP to the CO as well, either simultaneously or in different communication sessions. The same transmission line might be utilized for data transfer by both sites or the transmission to and from the CO might occur on two separate lines. In this regard reference is now directed to FIG. 1, which illustrates a prior art xDSL communication system 1. Specifically, FIG. 1 illustrates communication between a central office (CO) 10 and a customer premise (CP) 20 by way of twisted-pair telephone line 30. While the CP 20 may be a single dwelling residence, a small business, or other entity, it is generally characterized as having plain old telephone system (POTS) equipment, such as a telephone 22, a public switched telephone network (PSTN) modem 25, a facsimile machine 26, etc. The CP 20 may also include an xDSL communication device, such as an xDSL modem 23 that may permit a computer 24 to communicate with one or more remote networks via the CO 10. When a xDSL service is provided, a POTS filter 21 might be interposed between the POTS equipment 22 and the twisted-pair telephone line 30. As is known, the POTS filter 21 includes a low-pass filter having a cut-off frequency of approximately 4 kilohertz to 10 kilohertz, in order to filter high frequency transmissions from the xDSL communication device 23 and to protect the POTS equipment from the higher frequency xDSL equipment (e.g., the phone 22 and the facsimile machine 26 ).
At the CO 10, additional circuitry is typically provided. Generally, a line card (i.e., Line Card A) 18 containing line interface circuitry is provided to communicatively couple various xDSL service related signals along with PSTN voice signals to the twisted-pair telephone line 30. In fact, multiple line cards 14, 18 may be provided to serve a plurality of copper telephone subscriber loops. In the same way, additional interface circuit cards are typically provided at the CO 10 to handle different types of services. For example, an integrated services digital network (ISDN) interface card 16, a digital loop carrier line card 17, and other circuit cards, for supporting similar and other communication services, may be provided.
A digital switch 12 is also provided at the CO 10 and is configured to communicate with each of the various line cards 14, 16, 17, and 18. At a PSTN interface side of the CO (i.e., the side opposite the various line cards 14, 16, 17, and 18 supporting the telephone subscriber loops), a plurality of trunk cards 11, 13, and 15 are typically provided. For example, an analog trunk card 11, a digital trunk card 13, and an optical trunk card 15 are illustrated in FIG. 1. Typically, these circuit cards have outgoing lines that support numerous multiplexed xDSL service signal transmissions.
Having introduced a conventional xDSL communication system 1 as illustrated and described in relation to FIG. 1, reference is now directed to FIG. 2, which is a prior art functional block diagram further illustrating the various functional elements in a xDSL communications link 40 between a line card 18 located within a CO 10 and a xDSL modem 23 located at a CP 20 as introduced in FIG. 1. In this regard, the xDSL communications link 40 of FIG. 2 illustrates data transmission from a CO 10 to a CP 20 via a transmission line 30, such as, a twisted-pair telephone transmission line as may be provided by a POTS service provider to complete a designated link between a CO 10 and a CP 20. In addition, FIG. 2 further illustrates data transmission from the CP 20 to the CO 10 via the same twisted-pair telephone transmission line 30. With regard to the present illustration, transmission of data may be directed from the CP 20 to the CO 10, from the CO 10 to the CP 20 or in both directions simultaneously. Furthermore, data transmissions can flow on the same twisted-pair telephone transmission line 30 in both directions, or alternatively on separate transmission lines (one shown for simplicity of illustration). Each of the separate transmission lines may be designated to carry data transfers in a particular direction either to or from the CP 20.
The CO 10 may include a printed circuit line card 18 (see FIG. 1) that includes a CO-digital signal processor (DSP) 43, which receives digital information from one or more data sources (not shown) and sends the digital information to a CO-analog front end (AFE) 45. The CO-AFE 45 interposed between the twisted-pair telephone transmission line 30 and the CO-DSP 43 may convert digital data, from the CO-DSP 43, into a continuous time analog signal for transmission to the CP 20 via the one or more twisted-pair telephone transmission lines 30.
One or more analog signal representations of digital data streams supplied by one or more data sources (not shown) may be converted in the CO-AFE 45 and further amplified and processed via a CO-line driver 47 before transmission by a CO-hybrid 49, in accordance with the amount of power required to drive an amplified analog signal through the twisted-pair telephone transmission line 30 to the CP 20.
As is also illustrated in FIG. 2, the xDSL modem 23 located at the CP 20 may comprise a CP-hybrid 48. The CP-hybrid 48 may be used to de-couple a received signal from the transmitted signal in accordance with the data modulation scheme implemented by the particular xDSL data transmission standard in use. A CP-AFE 44, also located at the CP 20, may be configured to receive the de-coupled received signal from the CP-hybrid 48. The CP-AFE 44 may be configured to convert the received analog signal into a digital signal, which may then be transmitted to a CP-DSP 42 located at the CP 20. Finally, the digital information may be further transmitted to one or more specified data sources such as the computer 24 (see FIG. 1).
In the opposite data transmission direction, one or more digital data streams supplied by one or more devices in communication with the CP-DSP 42 at the CP 20 may be converted by the CP-AFE 44 and further amplified via CP-line driver 46. As will be appreciated by those skilled in the art, the CP-line driver 46 may amplify and forward the transmit signal with the power required to drive an amplified analog signal through the twisted-pair telephone transmission line 30 to the CO 10. It is significant to note that the CP-hybrid 48 is used to regenerate the transmit signal so it may be subtracted from the receive signal when the DSL communication system 1 is receiving at the CP 20. As a result, the CP-hybrid 48 does not affect the transmitted signal in any way. The CO-AFE 45 may receive the data from the CO-hybrid 49, located at the CO 10, which may de-couple the signal received from the CP 20 from the signal transmitted by the CO 10. The CO-AFE 45 may then convert the received analog signal into one or more digital signals, which may then be forwarded to the CO-DSP 43 located at the CO 10. Finally, the digital information may be further distributed to one or more specified data sources (not shown) by the CO-DSP 43.
Having briefly described a xDSL communications link 40 between the line card 18 located within the CO 10 and the xDSL modem 23 located at the CP 20 as illustrated in FIG. 2, reference is now directed to FIG. 3. In this regard, FIG. 3 is a prior art circuit schematic for a conventional hybrid 49.
As illustrated in FIG. 3, a transmit signal, TX, may be provided from the CO-Line Driver 47 (FIG. 2) and applied across a back-matching resistor 57, herein labeled, xe2x80x9cRb.xe2x80x9d As is further illustrated in FIG. 3, impedance and voltage scaling may be performed by coupling the transmit signal, TXxe2x80x2, to a two-wire transmission line, herein labeled, xe2x80x9cTIPxe2x80x9d and xe2x80x9cRINGxe2x80x9d via a transformer 59.
As is also illustrated in FIG. 3, the transmit signal, TX, may be applied to a scaled voltage divider consisting of a first filter 53, labeled, xe2x80x9cZbxe2x80x9d and a second filter 55, labeled, xe2x80x9cZm.xe2x80x9d The first filter 53 may be configured such that it emulates a scaled version of the back-matching resistor 57. For example, if the back-matching resistor is implemented with a resistor having a resistance of X Ohms, the first filter 53, Zb, may be implemented such that its equivalent impedance is nX Ohms. Similarly, the second filter 55, Zm, may be configured such that it emulates the sum of the line and load impedances, multiplied by the same scale factor, n. In a manner well known in the art, the transmit signal, TXxe2x80x2, may be echoed across the second filter 55 and may be subtracted from a duplex signal, VDUPLEX, comprising the combined receive and transmit signals, RXxe2x80x2 and TXxe2x80x2, respectively, appearing at the primary of the transformer 59 by a hybrid amplifier 61. As further illustrated in FIG. 3, the output of the hybrid amplifier 61, should comprise the received signal, RXxe2x80x3, from a remotely located transmitter after the transmit signal, TXxe2x80x2, has been subtracted.
In systems designated for data transmission over metallic transmission lines 30, the line driver amplifier 47 is the power amplifier which delivers the necessary energy to transmit a signal through the transmission line 30 through the back-matching resistor 57. The back-matching resistor 57 serves two purposes. First, the back-matching resistor 57 serves to match the impedance at the end of the transmission line 30. In order to provide a sufficient return loss, a resistor approximately equal to the transmission line""s 30 characteristic impedance must terminate the line. Second, the back-matching resistor 57 permits the hybrid 49 to simultaneously receive signals generated from a remote transmitter coupled to the transmission line 30 at the same time the line driver 47 is transmitting. The line driver 47 cannot terminate the transmission line 30 alone because the line driver 47 presents a low load impedance to the remotely transmitted signal, RX. As a result, using a line driver 47 alone would be the equivalent of shunting the remote signal to ground, thus making the receive signal, RX, unrecoverable. The remotely transmitted signal, RX, is recovered by subtracting from the voltage on the transmission line 30 (i.e., the duplex signal) the voltage introduced on the transmission line 30 by the local transmitter, TXxe2x80x2. As shown, the hybrid amplifier 61 performs the task of separating and recovering the remotely transmitted signal (i.e., the received signal) from the transmission line 30.
For simplicity of illustration and description the prior art hybrid circuit of FIG. 3 is depicted in a single-ended configuration. Those skilled in the art will appreciate that in practice a differential and balanced version of the hybrid 49 may be implemented. The hybrid 49 functions properly if the line driver 47 has a very low output impedance. From a data transmission viewpoint, the output of the line driver 47 is an amplified version of the transmit signal. This amplified version of the transmit signal, TX, is applied across a voltage divider comprising the back-matching resistor 57 and the primary winding of the transformer 59. As a result, a voltage corresponding to the amplified transmit signal is present on the primary of the transformer 59.
From a data receive viewpoint, a receive signal, RX, originating at a CP 20 may arrive at the secondary winding of the transformer 59. As is known, a corresponding receive signal voltage, RXxe2x80x2, is created via inductance on the primary winding of the transformer 59 and results in a current flowing into the back-matching resistor 57. Since the line driver 47 has a low output impedance, no component of the receive signal, RXxe2x80x2, is present at the output of the line driver 47, which leaves only the amplified transmit signal, TX, at the output of the line driver 47. Since the xDSL communication system 1 operates in a substantially linear fashion, superposition applies and the voltage across the primary winding of the transformer 59, VDUPLEX consists of both the receive, RXxe2x80x2, and the transmit signals, TXxe2x80x2.
If the first and second filters 53, 55 replicate the voltage divider formed by the back-matching resistor 57 and the primary winding of the transformer 59, then the voltage at the junction between the filter 53, 55 is equivalent to the voltage that would be applied across the transformer primary in the absence of a far end generated receive signal, TXxe2x80x2. As a result, the receive signal, RX, can be recovered by simply taking the difference between the voltage at the primary winding of the transformer 59 and the voltage at the junction between the first and second filters 53, 55. Hence, it is possible to simultaneously transmit and receive.
The prior art hybrid 49 circuit of FIG. 3 has the additional characteristic that components introduced by the line driver 47 are removed by the hybrid 49. In particular, transmit signal components due to imperfections in the line driver 47, such as noise and distortion, are removed by the hybrid and do not get forwarded to the CO-AFE 45 (FIG. 2) with the remotely generated receive signal. This functional aspect of the hybrid 49 is important because typically a high power amplifier, such as the line driver 47 amplifier, which provides the transmit signal will not be characterized by negligible noise and distortion at the required xDSL data transmission power levels.
The prior art hybrid 49 circuit of FIG. 3 suffers from the disadvantage that it is relatively inefficient. The voltage swing and power ultimately delivered to the primary winding of the transformer 59 and hence the secondary winding and the transmission line 30, is lower than the voltage swing and power sourced by the line driver 47. Assuming that the first and second filters 53, 55 have a sufficiently large and relatively matched impedance so that the power consumed within the filters 53, 55 is relatively negligible, a portion of the power delivered by the line driver 47 is dissipated in the back-matching resistor 57 with the remaining portion available at the primary winding of the transformer 59. That portion of the transmit signal dissipated in the back-matching resistor 57 can be reduced by reducing the magnitude of the resistance. However, the back-matching resistor 57 cannot be made arbitrarily small because the transmission line 30 would not be properly terminated at the primary winding of the transformer 59. Since the line driver 47 has a very small output impedance there would be no way of recovering the remotely generated receive signal, RX.
One way to avoid the power inefficiency inherent in the hybrid 49 presented in FIG. 3 is to construct a feedback circuit around the line driver 47 amplifier. Such a feedback circuit is presented in the circuit of FIG. 4. The circuit schematic presented in FIG. 4 and generally identified with reference numeral 60 is an example of a combination of a line driver 47 in cooperation with a positive feedback network and the transformer 59 of FIG. 3.
As illustrated in FIG. 4, a line driver amplifier with active termination 65 may be coupled with the transformer 59 of FIG. 3 to provide a transmit signal, TXxe2x80x2, at the primary winding of the transformer 59. This configuration may further provide an inductively coupled transmit signal, TXxe2x80x3, on a transmission line 30 that is electrically coupled to the secondary of the transformer 59. In this way, the line driver with active termination 65 appears as a voltage source at its output terminal with a low output impedance in series with a finite impedance. The apparent impedance may be adjusted such that the impedance matches the resistance of the back-matching resistor 57 of FIG. 3. The procedure of using feedback with an amplifier to generate an apparent impedance is generally known as active termination. The circuit schematic presented in FIG. 4 illustrates a relatively simple single-ended version of a line driver amplifier 47 with a positive feedback resistive network. For simplicity of illustration and description a single-ended version of the line driver with active termination 65 is presented. This presentation is by way of example only. Those skilled in the art will appreciate that a differential circuit implementation is typically selected to provide a line driver with active termination 65.
As illustrated in the exemplary circuit architecture of FIG. 4, the feedback network may comprise a plurality of individual components, typically resistors, generally configured as follows. A first resistor 71, herein labeled, xe2x80x9cR1,xe2x80x9d maybe interposed between an input or transmit signal terminal and the positive input terminal of the line driver power amplifier 47. A second resistor 73, labeled, xe2x80x9cR2,xe2x80x9d may be placed between the positive input terminal of the line driver power amplifier 47 and an output terminal of the line driver with active termination 65. A third resistor 75, labeled, xe2x80x9cRG,xe2x80x9d may be applied between signal ground and a negative input terminal of the line driver power amplifier 47. A fourth resistor 79, herein labeled, xe2x80x9cRxe2x80x2B,xe2x80x9d may be interposed between the output of the line driver power amplifier 47 and the output terminal of the line driver with active termination 65. A fifth resistor 77, labeled, xe2x80x9cRF,xe2x80x9d may be placed between the negative input terminal of the line driver power amplifier 47 and the output terminal of the line driver amplifier 47 as shown.
It can be further shown that as viewed from the primary winding of the transformer 59, the resistive network surrounding the line driver power amplifier 47 may cause the voltage across the primary winding to vary as a function of the current, IL, flowing through the primary winding, so that the primary winding appears to be driven by a voltage source through a finite impedance. With a suitable choice of resistance values for the various resistors 71, 73, 75, and 77, the apparent finite impedance can be shown to be the resistance value of the fourth resistor 79, (i.e., Rxe2x80x2B) multiplied by a factor given by the resistance values of the other resistors 71, 73, 75, and 77. Similarly, the equivalent line driver transmit gain of the line driver with active termination 65 (assuming an unloaded condition) may be determined in accordance with equation 1 using the resistance values for the various resistors 71, 73, 75, and 77.
More specifically, the equivalent line driver gain may be determined as follows:                               A          0                =                                            (                              A                -                K                            )                                      (                              1                -                K                            )                                .                                    Eq        .                  xe2x80x83                ⁢        1            
where,   A  =            (                        R          G                +                  R          F                    )              R      G      
and   K  =                    A        xc3x97                  R          1                            (                              R            1                    +                      R            2                          )              .  
Similarly, the apparent back-matching resistance may be determined from the following function:                               R          OUT                =                                            R              B              xe2x80x2                                      (                              1                -                K                            )                                .                                    Eq        .                  xe2x80x83                ⁢        2            
Since the apparent back-matching resistance is not implemented as a physical resistor, but rather by controlling the output voltage as a function of the output current, little power is dissipated and little signal swing is lost. In the limit, if the fourth resistor 79, Rxe2x80x2B, is implemented with a very low resistance value and the other resistors 71, 73, 75, and 77 are implemented to give the desired apparent resistance, virtually all the power from the line driver power amplifier 47 may be delivered to the primary winding of the transformer 59. In this case, the remotely generated receive signal, RX, sees the appropriate back-matching resistance (i.e., impedance) and the receive signal, RXxe2x80x2, can be recovered from the primary winding of the transformer 59.
The line driver with active termination 65 illustrated in FIG. 4 has several drawbacks. First, the feedback network in cooperation with the line driver power amplifier 47 uses positive feedback. This can be determined by examining the expressions for determining the equivalent line driver gain, A0, and the apparent back-matching resistance, ROUT. Observe that as the various resistance values are changed so that K approaches and then exceeds 1, the behavior of the circuit will qualitatively change as the signs (not only the magnitude) of the equivalent line driver gain, A0, and the apparent back-matching resistance, ROUT, change. These breaks or critical points in the functions defining both variables are characteristic of positive feedback systems. Positive feedback, in addition to introducing the qualitative changes noted above, also tends to emphasize component imperfections, system noise, and signal distortion.
Furthermore, a hybrid circuit cannot be connected to the line driver with active termination 65 illustrated in FIG. 4, because there is no node at which the voltage is due solely to the transmit signal other than the unamplified transmit signal input. The amplified transmit signal can be used to power the divider formed by the first and second filters 53, 55 (FIG. 3), or equivalent filters for that matter, which would lead to a recovery of the remotely generated receive signal at the primary winding of the transformer 59 (FIG. 3). However, in contrast to the conventional hybrid of FIG. 2, imperfections in the line driver amplifier 47 (FIG. 3) in the form of noise and distortion would be introduced only onto the primary winding of the transformer 59 (FIG. 3) and not onto the divider formed by the filters 53, 55 (FIG. 3). As a result, noise and distortion introduced by the line driver amplifier 47 would not be canceled out by the hybrid amplifier 61 (FIG. 3).
Finally, if it were desired to change or adjust the apparent back-matching impedance, ROUT, in order to compensate for variance in the manufacture of Rxe2x80x2B while attempting to maintain the equivalent line driver gain, A0, the various resistors 71, 73, 75, and 77 must be adjusted in a complicated way because the gain and back-matching impedance are not independent of one another.
Accordingly, there is a need for a line driver with improved power efficiency that can be used in cooperation with a hybrid to remove line driver generated signal imperfections and to recover a remotely generated receive signal from a duplex signal transmission on a transmission line.
In light of the foregoing, the invention is a circuit and a method for constructing a line driver having increased power efficiency capable of driving a data transmission line with a transmitted signal having no line driver amplifier generated signal imperfections and capable of cooperation with a hybrid to recover a remotely generated receive signal from a duplex signal transmission. The improved line driver architecture of the present invention uses a negative feedback control loop, thereby enhancing operational stability and suppressing both amplifier introduced imperfections and discrete component manufacturing variances. Furthermore, the improved line driver of the present invention provides a power efficient full duplex solution for line driver applications.
In a preferred embodiment, an improved line driver may comprise an active line termination control loop with a current sense feedback, a first amplifier, and a second amplifier. By integrating the improved line driver with a hybrid the composite circuit provides a scaled version of the transmit signal which is free from remotely generated or receive signal effects, as well as, imperfections due to noise and distortion. In addition, the composite circuit provides a power efficient solution through the use of a finite and independently adjustable output impedance that may be used to avoid some of the loss in signal power that is typically dissipated within the line termination (i.e., the back-matching) resistance.
The present invention can also be viewed as providing a method for increasing the stability, power efficiency, and accuracy of a line driver. In its broadest terms, the method can be practiced by performing the following steps: applying a transmit signal to an input of a line driver; amplifying the transmit signal; applying the amplified transmit signal to a transmission line load to generate a load current; sensing the load current; and applying the sensed load current in a negative feedback control loop to generate a feedback signal responsive to the load current such that an output impedance that emulates a back-matching resistor is generated.
The present invention can be further viewed as providing a method for recovering a remotely generated signal from a transmission line in a duplex communication system. In its broadest terms the method can be practiced by performing the following steps: applying a transmit signal to an input stage of a line driver; amplifying the transmit signal; using an active termination feedback control loop to generate a feedback signal; amplifying the feedback signal; combining the feedback signal with a duplex signal on a transmission line to generate a scaled transmit signal; and combining the scaled transmit signal with the duplex signal to recover a remotely generated receive signal from the transmission line.
Other features and advantages of the present invention will become apparent to one skilled in the art upon examination of the following drawings and detailed description. It is intended that all such additional features and advantages be included herein within the scope of the present invention, as defined by the claims.